I know TID numbers are only an "indication" but I seem to be getting a fair few finds with TID's in the 50's and up that turn out to be rusted "something"...I would have thought anything capable of rusting so badly would have a low TDI?

The machine in question is a Racer 2 and it was on two tone with very little discrm dialled in..

Also, how do TID's vary between different brands of detector? For instance, I believe 4 is generally ferrous which around 8 to 10 is often foil??

Late as usual but I thought to give what I think is happening, it is averaging up the ID number. The rust layers in the ground is enhancing or making the carbon content read higher than normal.360 the target and see if it changes, a good target ID will jump noticeably higher in one direction.

The more I use the machine, the better I am getting at interpreting whats going on..So far I would say that targets with fluctuating TID tend to be trash such as rusty scrap where as "pure" targets such coins, buttons or musket balls tend to have fairly constant targets..I am also noticing the same with the associated tones...

Coke seems to be a bit of a wild card with some what variable TID's, same with aluminium, which I think is down to the various alloy's you can find it in, along with its "form" ie thin foil is often different to the thicker stuff which is different again to ring pulls....